Secretion across the inner membrane in some Gram-negative bacteria occurs via the preprotein translocase pathway. Proteins are produced in the cytoplasm as precursors, and require a chaperone subunit to direct them to the translocase component []. From there, the mature proteins are eithertargeted to the outer membrane, or remain as periplasmic proteins []. The translocase protein subunits are encoded on the bacterial chromosome. The translocase itself comprises 7 proteins, including a chaperone (SecB), ATPase (SecA), an integral membrane complex (SecY, SecE and SecG), and two additional membrane proteins that promote the release of the mature peptide into the periplasm (SecD and SecF) []. Other cytoplasmic/periplasmic proteins play a part in preprotein translocase activity, namely YidC and YajC []. The latter is bound in a complex to SecD and SecF, and plays apart in stabilising and regulating secretion through the SecYEG integralmembrane component via SecA []. Homologues of the YajC gene have been found in a range of pathogenic and commensal microbes. Brucella abortis YajC- and SecD-like proteins were shownto stimulate a Th1 cell-mediated immune response in mice, and conferred protection when challenged with B.abortis []. Therefore, these proteins mayhave an antigenic role as well as a secretory one in virulent bacteria [].A number of previously uncharacterised "hypothetical" proteins also showsimilarity to E.coli YajC, suggesting that this family is wider than first thought []. More recently, the precise interactions between the E.coli SecYEG complex, SecD, SecF, YajC and YidC have been studied []. Rather than acting individually, the four proteins form a heterotetrameric complex and associate with the SecYEG heterotrimeric complex []. The SecF and YajC subunits link the complex to the integral membrane translocase.

This database is part of a project that received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement number 649024).